A

A remote-sensing sys­tem (e.g., an instrument) that transmits its own radi­ation to detect an object or area for observation and receives the reflected or transmitted radiation. Radar is an example of an active system. Compare with passive sensor.

B

C

Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elements in a given region over a long period of time. Climate can be contrasted to weather, which is the present condition of these same elements and their variations over shorter time periods.

E

The sum of evaporation and plant transpiration. Potential evapotranspiration is the amount of water that could be evaporated or transpired at a given temperature and humidity, if there was plenty of water available. Actual evapotranspiration can not be any greater than precipitation, and will usually be less because some water will run off in rivers and flow to the oceans. If potential evapotranspiration is greater than actual precipitation, then soils are extremely dry during at least a major part of the year.

Graupel, also called soft hail or snow pellets, refers to precipitation that forms when supercooled droplets of water condense on a snowflake, forming a 2–5 mm ball of rime; the snowflake acts as a nucleus of condensation in this process. The term graupel is the German word for this meteorological phenomenon. Graupel is sometimes referred to as small hail, although the World Meteorological Organization defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail.

H

Precipitation composed of balls or irregular lumps of ice. Hail is produced when large frozen raindrops, or almost any particles, in cumulonimbus clouds act as embryos that grow by accumulating supercooled liquid droplets. Violent updrafts in the cloud carry the particles in freezing air, allowing the frozen core to accumulate more ice. When the piece of hail becomes too heavy to be carried by upsurg­ing air currents it falls to the ground.

All of Earth's water, including surface water (water in oceans, lakes, and rivers), groundwater (water in soil and beneath the Earth's surface), snowcover, ice, and water in the atmosphere, including water vapor.

M

A type of electromagnetic energy; the portion of electromagnetic energy between radio waves and infrared radiation, which is often measured by scientific instruments to determine atmospheric conditions from space.

N

When an object, such as a satellite, is circling the Earth at such a speed that it crosses over the same location at different times of the day. This allows the satellite to collect measurements during different solar conditions (i.e. morning, evening, night).

In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena) is any product of the condensation of atmospheric water vapor that falls quickly out of a cloud. This is in contrast to cloud water, which is ice or liquid water that falls slowly enough that it can remain in the air for hours. The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail. It occurs when a local portion of the atmosphere becomes saturated with water vapour and the water condenses.

An instrument for quantitatively measuring the intensity of electromagnetic radiation in some band of wavelengths in any part of the electromagnetic spectrum. Usually used with a modifier, such as an infrared radiometer or a microwave radiometer.

A measure of the fraction of incident radiation falling on a surface that is turned back from it by reflection. Reflectivity also refers to the degree by which precipitation is able to reflect a radar beam.

Runoff occurs when falling rain cannot be absorbed by soil, and thus instead of going into the ground the water flows over the surface. It occurs in nature when the soil is saturated with water, or in urban environments when there is asphalt instead of soil on the ground.